This application generally relates to nuclear reactor fuel assemblies and more particularly relates to a nuclear fission reactor fuel assembly and system configured for controlled removal of a volatile fission product and heat released by a burn wave in a traveling wave nuclear fission reactor and method for same.
It is known that, in an operating nuclear fission reactor, neutrons of a known energy are captured by nuclides having a high atomic mass. The resulting compound nucleus separates into fission products that include two lower atomic mass fission fragments and also decay products. Nuclides known to undergo such fission by neutrons of all energies include uranium-233, uranium-235 and plutonium-239, which are fissile nuclides. For example, thermal neutrons having a kinetic energy of 0.0253 eV (electron volts) can be used to fission U-235 nuclei. Fission of thorium-232 and uranium-238, which are fertile nuclides, will not undergo induced fission, except with fast neutrons that have a kinetic energy of at least 1 MeV (million electron volts). The total kinetic energy released from each fission event is about 200 MeV. This kinetic energy is eventually transformed into heat.
Moreover, the fission process, which starts with an initial source of neutrons, liberates additional neutrons as well as transforms kinetic energy into heat. This results in a self-sustaining fission chain reaction that is accompanied by continued release of heat. For every neutron that is absorbed, more than one neutron is liberated until the fissile nuclei are depleted. This phenomenon is used in a commercial nuclear reactor to produce continuous heat that, in turn, is used to generate electricity.
Attempts have been made to address fission product accumulation during reactor operation. U.S. Pat. No. 4,285,891, issued Aug. 25, 1981 in the names of Lane A. Bray et al. and titled “Method of Removing Fission Gases from Irradiated Fuel” discloses a method for removing volatile fission products from irradiated fuel by first passing a hydrogen-containing inert gas by the fuel which is heated to an elevated temperature of at least 1000° C. and then passing inert gas alone by the fuel which is at the elevated temperature.
Another approach is disclosed in U.S. Pat. No. 5,268,947, issued Dec. 7, 1993 in the names of Bernard Bastide et al. and titled “Nuclear Fuel Elements Comprising a Trap for Fission Products Based on Oxide”. This patent discloses a nuclear fuel element comprising sintered pellets which are surrounded by a metallic sheath and permitting trapping of the fission products characterized in that the pellets contain or are coated with or that the sheath is internally coated with an agent for trapping the fission products. The fission products are trapped by forming with the trapping agent oxygenated compounds which are stable at high temperature.